Sewing machines of the lockstitch type conventionally employ a rotary hook disposed beneath the bed of the machine for cooperation with the needle when the latter penetrates the material and passes through the needle opening in the throat plate of the bed. In such machines the rotary hook carries a thread bobbin which supplies the thread for interengagement with the thread carried by the needle. The thread bobbin is enclosed in a bobbin case, removably supported upon the center pin or stud of the bobbin case holder and retained in place by a latching means.
Bobbin cases of the type mentioned above also conventionally employ a thread tensioning means for applying a frictional force or tension upon the thread coming from the bobbin. Where a latching means, as mentioned above, is utilized upon the bobbin case, the tensioning means ordinarily is provided upon the side wall of the case. The tension means is normally a spring secured by a screw to the side wall of the bobbin case to which the tension can be adjusted by means of an adjusting screw. The bobbin thread extends between the outside of the side wall of the bobbin case and the underside of the tension spring. As the adjusting screw is tightened, more pressure is exerted on the thread. This tension means is very effective on small size threads. But when using large size threads, such as size 99, 138 and 207 nylon its effectiveness decreases. A somewhat light tension can be held, but when tightening the adjusting screw, the tension increases, but is very erratic, that is not uniform. This results in the making of an unbalanced stitch, namely bobbin thread being pulled up to be seen from the top or needle thread loops hanging down from the underside.